1. Field of the Invention
The present invention relates to an electronic component including a plurality of resonators provided within a layered substrate.
2. Description of the Related Art
There are strong demands for reductions in size and thickness of communication apparatuses for short-range wireless communications, such as communication apparatuses conforming to the Bluetooth standard and communication apparatuses for use on a wireless local area network (LAN). Accordingly, reductions in size and thickness are also demanded of electronic components incorporated in such communication apparatuses. A bandpass filter that filters reception signals is one of electronic components incorporated in the communication apparatuses mentioned above. Reductions in size and thickness are also demanded of the bandpass filter. To meet the demands, a layered filter including a plurality of resonators each formed using at least one conductor layer of a layered substrate has been proposed as a bandpass filter that is operable in the frequency bands used for the above-mentioned communication apparatuses and capable of achieving reductions in size and thickness. Such a layered filter is disclosed in, for example, JP-A-9-148802, JP-A-2001-119209, JP-A-2005-012258 and JP-A-2005-159512. Hereinafter, a conductor layer used for forming a resonator is referred to as a resonator-forming conductor layer.
JP-A-9-148802 discloses a layered bandpass filter including at least two resonators. In this bandpass filter, each of the resonators incorporates two types of internal electrodes that are alternately arranged in the stacking direction and that each have a short-circuited end and an open-circuited end whose relative positions are reversed between the two types.
JP-A-2001-119209 discloses a layered filter module including a plurality of filters, each of the filters including a plurality of inductor-forming conductors. Each of the filters of this module incorporates three resonators formed using the inductor-forming conductors. In this module, the inductor-forming conductors in every adjacent filters do not include portions extending in parallel with each other along the entire length.
FIG. 7 of JP-A-2005-012258 shows a bandpass filter including four resonators. In this bandpass filter, each of the resonators incorporates two types of capacitance-forming electrodes that are alternately arranged in the stacking direction and that each have a short-circuited end and an open-circuited end whose relative positions are reversed between the two types. FIG. 1 of this publication shows a bandpass filter including three resonators Q1, Q2 and Q3. In this bandpass filter, the resonators Q1, Q2 and Q3 incorporate their respective strip lines for inductors. The strip lines of the resonators Q1 and Q2 are combline-coupled to each other, while the strip lines of the resonators Q2 and Q3 are interdigital-coupled to each other.
JP-A-2005-159512 discloses a layered bandpass filter including three resonator electrodes arranged side by side on one dielectric layer. The three resonator electrodes of this bandpass filter are disposed in a combline form or an interdigital form.
Typically, a bandpass filter including a plurality of resonators exhibits a broader passband width and a steeper attenuation pole as the number of the resonators increases.
For a conventional layered bandpass filter including a plurality of resonators, it is required to reduce the distance between every adjacent resonators in order to achieve reductions in size and thickness. If this is done, however, the inductive coupling between every adjacent resonators becomes too strong, so that it becomes difficult to attain desired filter characteristics. Specifically, the passband width of the filter becomes too broad if the inductive coupling between adjacent resonators becomes too strong.
For reducing the inductive coupling between every adjacent resonators in a layered bandpass filter without interfering with reductions in filter size and thickness, a possible approach is to reduce the width of each resonator-forming conductor layer to thereby increase the distance between every adjacent resonators. However, this reduces the Qs of all of the resonators.
To increase the resonator Q, it is effective to increase the surface area of the resonator-forming conductor layer. In view of this, each resonator can be formed using a plurality of resonator-forming conductor layers so as to increase the distance between every adjacent resonators to some extent without reducing the resonator Q. In this case, each resonator can be formed of resonator-forming conductor layers of two types that are alternately arranged in the stacking direction and that each have a short-circuited end and an open-circuited end whose relative positions are reversed between the two types, as proposed in JP-A-9-148802 or JP-A-2005-012258. In this case, the resonator-forming conductor layers of the two types alternately arranged in the stacking direction are interdigital-coupled to each other, thereby constituting a resonator including an inductor and a capacitor.
In the layered bandpass filter disclosed in JP-A-9-148802, a lead-out electrode connected to an external electrode for input/output is electrically coupled to one of the internal electrodes by capacitive, inductive or direct coupling. In the bandpass filter disclosed in FIG. 7 of JP-A-2005-012258, a lead-out electrode connected to an input terminal electrode and a lead-out electrode connected to an output terminal electrode are each connected to one of the capacitance-forming electrodes. Thus, JP-A-9-148802 and JP-A-2005-012258 teach that, of the resonator-forming conductor layers of the two types that are alternately arranged in the stacking direction and that are reversed in relative positions of the short-circuited end and the open-circuited end, a single resonator-forming conductor layer of one of the two types connects to a terminal for input or output of signals. This configuration cannot greatly increase the Q of the resonator connected to the terminal for input or output of signals.